1. Field of the Invention
The present invention relates to an optical device for compensating chromatic dispersion in optical telecommunications systems.
2. Technical Background
The optical fibers used for transmitting signals in optical telecommunications systems show a phenomenon called chromatic dispersion, due to the combination of the characteristics of the constituent material of these fibers and the characteristics of their refractive index profile; this chromatic dispersion is variable with the wavelength of the signals transmitted and is canceled at a certain value of wavelength.
This phenomenon of chromatic dispersion essentially consists of a widening of the duration of the pulses forming the signal during transit through the fiber, this widening being due to the fact that the different chromatic components of each pulse, each characterized by its own wavelength, travel at different velocities in the fiber.
As a result of this widening, pulses which follow each other in time and are quite distinct from each other at the moment of transmission can become partially superimposed on reception, after traveling along the fiber, up to the point where they are no longer distinguishable as separate entities, causing an error in reception.
Chromatic dispersion can be reduced by using, in place of the ordinary SI (step index) fibers, which have a zero dispersion at a wavelength in the region of 1300 nm, transmission fibers in which the chromatic dispersion cancellation point is shifted, these being known as DS (dispersion shifted); the optical characteristics of these fibers are designed in such a way that the chromatic dispersion cancellation point is brought to a wavelength in the region between 1500 and 1600 nm, which is commonly used for telecommunications.
Fibers of this kind are defined in ITU-T Recommendation G.653 of March 1993, which specifies that the chromatic dispersion of the fiber should be canceled nominally at a wavelength xcex0 of 1550 nm, with a tolerance of 50 nm with respect to this value.
DS fibers are described, for example, in U.S. Pat. Nos. 4,715,679, 4,822,399, and 4,755,022.
Even when transmission fibers of the DS type are used, however, the signals will inevitably undergo a residual chromatic dispersion when propagated through long cable runs, of the order of hundreds of kilometers for example, because of the variations of the chromatic dispersion cancellation wavelength about the nominal value over the length of the fiber.
In the case of optical transmission of the wavelength division multiplexing (or WDM) type, in which signals at different wavelengths are transmitted simultaneously along the line, a positive or negative chromatic dispersion is produced for signals at wavelengths above or below the chromatic dispersion cancellation wavelength.
In-fiber Bragg gratings are formed by an alternation of areas having a high refractive index with areas having a low refractive index. The distance between these areas is called the pitch of the grating. The pitch of the grating determines which wavelengths are reflected and which are transmitted. Patent application WO9636895 describes a method for writing this type of grating in an optical fiber. To compensate the chromatic dispersion, a proposal was made in an article by F. Ouellette, published in Optics Letters, Vol. 12, No. 10, pp. 847-849, October 1987, and in U.S. Pat. No. 4,953,939, in the name of Epworth, of Apr. 9, 1990, to use an optical fiber with distributed Bragg reflection with a variable-pitch grating (chirped grating).
The article xe2x80x9cDual on fiber thin-film heaters for fiber gratings with independently adjustable chirp and wavelengthxe2x80x9d, published in Optics Letters, Vol. 24, No. 19, Oct. 1, 1999, describes a chirped in-fiber Bragg grating on whose external surface there is a first metallic coating on which a second metallic coating is superimposed. There is an insulating layer between the two metallic coatings. An electric current is applied to each of the metallic coatings. The Bragg wavelength and the chirping factor can be controlled by controlling the intensity of these currents.
Patent application WO9726581 describes a Bragg grating fitted on a dimorphous element. In response to an electrical control signal, this dimorphous element bends in such a way as to modify the spectral response of the Bragg grating. Such a Bragg grating fitted on such a dimorphous element is used as a chromatic dispersion compensator.
The applicant has observed that, in this patent application, the torsion of the dimorphous element causes a modification of the pitch of the grating of the linear type; in other words, as the dimorphous element is bent, the pitch at any point of the grating is modified by the same amount.
For wavelength division multiplexing, or WDM, transmission, a plurality of mutually independent transmission signals have to be sent along the same line, consisting of optical fibers, by means of multiplexing in the optical wavelength domain; the transmitted signals can be either digital or analog, and are distinguished from each other in that each of them has a specific wavelength, separate from that of the other signals.
To implement this WDM transmission, specific wavelengths of predetermined amplitude, termed xe2x80x9cchannelsxe2x80x9d in the following text, have to be assigned to each of the signals at different wavelengths. These channels, each identified in the following text by a wavelength value, called the central channel wavelength, have a certain spectral amplitude around the central wavelength value, which depends, in particular, on the characteristics of the signal source laser and on the modulation imparted to this to associate a data element with the signal. Typical values of spectral amplitude of the signal emitted by a laser, in the absence of modulation, are in the region of 10 MHz; in the presence of external modulation, at 2.5 Gbit/s for example, the spectral amplitude is approximately 5 GHz.
In order to transmit signals in a large number of channels, making use of what is known as the third transmission window of silica fibers and of the bandwidth of optical amplifiers (for example, from 1525 to 1565 nm, or from 1540 to 1620 nm, or from 1525 to 1620 nm), the wavelength separation between the channels is conveniently of the order of nanometers or fractions of nanometers.
For correct reception of these transmission signals, it is necessary to provide a separation between the signals, for directing them to the corresponding users. Furthermore, during their travel along the line the signals can undergo alterations due to the said phenomenon of chromatic dispersion; moreover, signals having different wavelengths from each other can undergo different alterations from each other, and consequently some channels have a better transmission quality than others following demultiplexing.
The applicant has tackled the problem of compensating the effects of chromatic dispersion in multiple-wavelength telecommunications systems, in other words in systems in which signals at different wavelengths are transmitted simultaneously along the line (WDM). In these systems, the chromatic dispersion is different for each channel, and therefore, in order to compensate the effects of this phenomenon accurately, it is advantageous to use a dispersion compensator for each channel of the multiple-wavelength signal after the WDM signal has been demultiplexed. The applicant has considered the problem of making a component capable of compensating the chromatic dispersion in a multiple-wavelength system.
The applicant has found that it is possible to compensate the chromatic dispersion of each channel of a multiple-wavelength signal by forming a variable-pitch Bragg grating and modifying the said variable pitch appropriately according to the channel, by means of a distributed elongation of the said grating. In particular, the applicant has found that, by fixing the said grating, made in an optical fiber for example, to a supporting substrate, which undergoes a non-linear elongation effect, it is possible to compensate the effects of chromatic dispersion on a channel of the said WDM signal. The form of the said substrate is such that it undergoes a non-linear elongation, with a consequent equivalent elongation of the grating. For each channel of the multiple-wavelength signal an elongation is predetermined, and is dependent on the central wavelength of the channel.
In one of its aspects, the present invention relates to an optical device for compensating chromatic dispersion, comprising a variable-pitch Bragg grating having a predetermined reflection wavelength and a predetermined chirping factor, and a substrate, on which this grating is integrally mounted, and which can be elongated in such a way as to cause a change in the said predetermined chirping factor.
In particular, the said grating is fitted into a longitudinal central groove of the said substrate. Preferably, the said substrate comprises a central portion essentially of truncated conical shape, having the inclined sides curved according to a predetermined profile of curvature.
Preferably, the said profile of curvature is a hyperbolic profile.
Preferably, the said Bragg grating is formed in an optical fiber.
The said optical device additionally comprises a traction device for elongating the said substrate. In particular, the said substrate comprises a first portion and a second portion in which are formed holes which can interact with the said traction device for elongating the said substrate.
Further characteristics and advantages of the present invention are disclosed in greater detail by the following description, with reference to the attached drawings, which are supplied solely for explanatory purposes and without any restrictive intent, and which show the following: